I. Field of the Invention
The present invention relates to a tensionless leveller intended for levelling a metal strip and to the levelling method using said leveller.
II. Description of Related Art
A metal strip or plate undergoes various operations, such as hot rolling and cold rolling, which are intended to give it uniform dimensions over its entire length. Thus, in theory a rolled metal strip has at any point a constant thickness and a constant width.
However, the rolling operation is insufficient for obtaining a defect-free strip. This is because it exhibits non-developable flatness defects, such as waviness at the edges or the center, and/or developable defects such as a curl or a crown, that is to say a curvature either along the length or along the width of the strip, respectively.
These flatness defects can be corrected by levelling the strip in a multi-roll leveller. Such a leveller consists of two superposed cassettes each supporting several motor-driven rolls, of constant diameter, offset with respect to one another and placed alternately above and below the path of the strip. This type of leveller is configured, in terms of the number of rolls, the diameter of the rolls, the center-to-center spacing and the setting, so as to achieve satisfactory levelling of the strip, the thickness of which lies within a defined range.
In a conventional leveller, the center-to-center spacings of the rolls are constant and set so that the ratio of the roll diameter to the center-to-center spacing is between about 0.90 and about 0.95. However, in this type of leveller, the levelling forces and moments are large. For the purpose of reducing them, manufacturers have developed levellers in which all of the center-to-center spacings are increased so that the ratio of the diameter to the center-to-center spacing is around 0.70 to 0.80. However, this no longer allows the non-developable defects to be corrected over the entire range of the leveller in terms of strip thickness, and in particular on a thinner strip.
Manufacturers have also proposed retracting some of the rolls, for example going from nine rolls to five. However, when the number of useful rolls is reduced, the degree of plastic deformation within the leveller varies abruptly, and it becomes difficult to bring the developable defects under control.